wave period

For discussion of surf forecasting science and technology, wave science, weather and board mechanics or for any questions about the magicseaweed.com forecasts and software.

wave period

Postby efoo » Thu Jun 30, 2005 12:55 pm

The statement about wave period in the description of swell period is incorrect. The author states that "longer period=faster waves". Period is equal to the wavelength divided by the speed of the wave which contradicts this statement. The period is the time taken for any point on the wave to return to its original position for example the time taken for a cork to return to its starting position after a wave has past, or the time between troughs or crests.
The author states "swell period is the distance between waves in a set". It is the time not the distance, i think he is confused about wavelength and period as a greater wavelength for the same period equates toa faster wave. Can anyone dispute this?
efoo
 
Posts: 4
Joined: Thu Jun 30, 2005 12:36 pm

Postby efoo » Thu Jun 30, 2005 2:13 pm

Thanks Benno, just to let you know i'm all for this site (its got some excellent info I've never seen pooled in the one spot before), i was merely offering some constructive criticism and trying to clarify in my own mind what i learnt in high school physics. One thing they didnt teach us was linerised wave theory but based on the 1.56xT equation, the simple wave relationship I mentioned is either irrelevant in deep water or the wavelength remains constant for any period or speed, if you can clarify which one I would be interested to know.
Whens the ozzie portion of the site due for completion?
efoo
 
Posts: 4
Joined: Thu Jun 30, 2005 12:36 pm

Postby purpleandy » Thu Jun 30, 2005 10:50 pm

ben wrote:As the author I understood that Linearized Wave Theory suggested that in deep water wave speed was equal to 1.56 x Period.

high school physics (ahhh - the wave equation) states that v = f x λ. if speed is 1.65 x frequency then the theory ben quoted could be interpreted as giving a fixed wavelength of 1.56m. a longer period = lower frequency and therefore a slower speed. All worked out using high school physics.
however, the theory ben has used is not high school physics. water waves are not progressive waves as you were taught (ie a wave caused either by a short term or fixed frequency disturbance at 0 degrees, 90 degrees or 270 degrees to the direction of propogation,). Water waves are affected by a multitude of external factors (water depth, wind speed, density etc etc). Now i'm not for a minute professing to know the slightest hint of the theory ben uses, but if the wind hits the water at any angle high school physics breaks down. infact, high school physics is buggered unless it involves angles of 0, 90, 180, 270 or 360 degrees. Any other angle means the maths gets horrendous. especially with waves, as if the driving force hits the water at any angle other than those listed above, it will create longitudinal and transvere waveforms in the same space, creating superposition and beat frequencies. Each of these will probably creatle localised currents in the water (gravity will try to pull the top of the wave down, and i reckon this means that the peaks will be accelerating when the troughs are travelling at constant velocity). This will continually affect the shape of the surface, setting up vortexes and eddy currents in the airflow. and its the airflow that creates the wave. Huuuuge feedback going on........or in english a bloody complicated wave. meaning that the 'clean' , ideal and simplistic wave equation that is universally taught breaks down due to the complex disturbance caused. the actual equation ben used (and i can't argue for or against) is velocity equals 1.56 x period. That means that as the constant is 1.56, the velocity is directly proportional to the period, hence a longer period means faster waves. its the theory ben has quoted and if it works and it's simple its usually right........therefore i'd go for bens theory.
User avatar
purpleandy
 
Posts: 1827
Joined: Thu Jan 01, 1970 12:00 am
Location: Northland, NZ

Postby purpleandy » Thu Jun 30, 2005 10:54 pm

p.s - if any of you on here studying surf science ( and i know there are a few) want to pull holes in my lickle argument........ :lol: :lol: ....... it'd be cool to hear....
just clicked the second lick and found the following quote 'The water particle velocities under linear waves are maximum at the surface and decrease in magnitude with depth. In shallow water, the elliptical paths followed by the water particles flatten to horizontal lines, particularly at the bottom, where no vertical flow is allowed into the bottom. ' backing up my theory that water waves are a mix of transverse and longitudinal waves......meaning that the standard wave equation is no longer relevant. hope this helps :?
User avatar
purpleandy
 
Posts: 1827
Joined: Thu Jan 01, 1970 12:00 am
Location: Northland, NZ

Postby mouse » Fri Jul 01, 2005 5:38 am

my brain hurts
with fins instead of hands
mouse
 
Posts: 1117
Joined: Sat Apr 17, 2004 10:59 am
Location: muriwai

Postby blind » Fri Jul 01, 2005 6:57 am

mouse wrote:my brain hurts
:shock: :shock: And me mouse , I just look at it as some magical thing that happens and they are made by wave fairys :wink:
Last edited by blind on Fri Jul 01, 2005 10:39 am, edited 1 time in total.
User avatar
blind
 
Posts: 1298
Joined: Thu Jan 01, 1970 12:00 am
Location: Plymouth

Postby efoo » Fri Jul 01, 2005 8:19 am

It appears i am a little out of my depth, pardon the pun, but i'm glad to see Ben changed the distance remark to time, the statement that initially confused me. Thanks for all your replies you have enlightened me incredibly to the point where my next backhand cuttie will be followed by ponderings about the vortexes and particle velocities i'd just created. One thing i'm still wondering though, doesnt the intrinsic relationship T=λ / ν or ν = f λ hold true for any wave because all it is stating is that the period or the time for a wave to pass is dependent on two factors, the length of that wave and the speed at which it is travelling? There may be external factors that affect the shape, direction, currents - creating a mega complex formula to describe the wave but essentially if a wave is travelling at 1 metre per second and has a wavelength of one metre then it will take one second to pass. Please help me or i wont sleep tonight.....
efoo
 
Posts: 4
Joined: Thu Jun 30, 2005 12:36 pm

Postby Cornerhouse » Fri Jul 01, 2005 8:47 am

All those wordy words :? :? :?

blind wrote:.......some magical thing that happens and the are made by wave fairys :wink:

I'm with you on that :)
User avatar
Cornerhouse
 
Posts: 292
Joined: Thu Jan 01, 1970 12:00 am
Location: North Devon

Postby efoo » Fri Jul 01, 2005 9:29 am

Flat as Benno, the Thames don't give two shits about period and wavelength so i'm a fish outta water here. My time in London is spent planning future trips and getting into overcomplicated discussions about wave fairy physics. Someone buy cornerhouse an oxford...
efoo
 
Posts: 4
Joined: Thu Jun 30, 2005 12:36 pm

Postby purpleandy » Fri Jul 01, 2005 10:32 am

i'm retiring from the argument as well - its obviously more complex. i thoght i had is that maybe the 1.56 thing is a model people use as it best approximates experimental data????? models and equations are two different things.......
User avatar
purpleandy
 
Posts: 1827
Joined: Thu Jan 01, 1970 12:00 am
Location: Northland, NZ

Postby surfrat » Fri Jul 01, 2005 10:46 am

all the wave fairies have gone on holiday to the costa del sol round here :(

can we borrow some of yours Blind?
http://www.facebook.com/pages/HunterSurf/133748279996238
User avatar
surfrat
 
Posts: 3087
Joined: Thu Mar 03, 2005 10:02 am
Location: Woolacombe

Postby EJay » Fri Jul 01, 2005 12:43 pm

O.K.!!! This is one of my first year lectures... have a look and can always explain more. And if thats not enough - i've got plenty more where this came from!

This ones prob just for Effo - it is the summer holidays after all!!!

WAVES IN DEEP WATER

In this lecture, we are concerned with waves travelling in DEEP water, that is water which is deep compared to their wavelength. For example, waves of length 100m travelling in an ocean of depth 1000m would be deep water waves. Similarly, waves of length 50cm travelling in a pond 20m deep would be deep water waves.

WAVE ORBITALS IN DEEP WATER

In waves in deep water, unlike those in shallow water, the water particles go round in circles, which get smaller with depth below the surface. The name given to these circles is ‘orbitals’. At the surface, the diameter of the circle equals the wave height.

HOW THE ORBITALS DECREASE WITH DEPTH

The diameter of the circles decreases quickly below the surface. To all intents and purposes there is no movement below a depth of one wavelength below the surface. The table below shows the size of the orbital at various depths. In the table, the depth is expressed in fractions of a wavelength, L, and the size of the orbital in fractions of the wave height, H.

For this reason, submarines can dive to avoid the effects of storms.

THE SPEED OF WAVES IN DEEP WATER

Later, we derive the speed of the waves from first principles. Here is the answer:


c = (gL/2ï
User avatar
EJay
 
Posts: 122
Joined: Thu Jan 01, 1970 12:00 am
Location: Sunny Wales!

Postby Grizzly » Fri Jul 01, 2005 1:42 pm

EJay wrote:DID YOU REALLY GET THIS FAR!!!!!!



Don't stop now, was enjoying that :D :D
Gorau dial, dangos cwm a'i ffadau.
http://singlefin.blogspot.com
User avatar
Grizzly
 
Posts: 1941
Joined: Thu Jan 01, 1970 12:00 am
Location: Aberystwyth

Postby sequoiaboard » Wed Jul 06, 2005 7:43 pm

bloody hell.....i feel like a complete simpleton now, i'll go with the wave fairy theory
sequoiaboard
 
Posts: 530
Joined: Thu Jan 01, 1970 12:00 am
Location: preferably in the mud

Postby blind » Wed Jul 06, 2005 8:22 pm

ok so a few of you believe in the wave fairys like me :D anybody up for the flat earth theory :? :? :? cos the fairys do disappear over the edge every so often , hence flat spells :lol: :lol: :lol:
User avatar
blind
 
Posts: 1298
Joined: Thu Jan 01, 1970 12:00 am
Location: Plymouth

Postby purpleandy » Wed Jul 06, 2005 9:44 pm

i get to teach the flat earth theory 8)
anyway, lets be sensible - we all know that flat spells aren't caused by the wave fairys dropping off the edge. That's just bloody ridiculous.
They've clearly been out the night before and are too hungover / stoned to make waves.......Let's think about this. Ever wondered why its flat in the summer? It's because all the wave fairies are too busy getting pissed and mashed at their mates BBQ's.........
User avatar
purpleandy
 
Posts: 1827
Joined: Thu Jan 01, 1970 12:00 am
Location: Northland, NZ

Postby blind » Thu Jul 07, 2005 6:49 am

Fair one Andy, you being a teacher should know ,I stand corrected :wink:
User avatar
blind
 
Posts: 1298
Joined: Thu Jan 01, 1970 12:00 am
Location: Plymouth

Postby Bigjohn » Mon Aug 08, 2005 3:49 pm

So that must mean that british wave fairies dont like the french ones cos the closer you get to france, the less waves you get :(
......Rubber Duck this is Pig Pen, I'm gonna put the hammer down..........

Image
Bigjohn
 
Posts: 4846
Joined: Thu Jan 01, 1970 12:00 am
Location: Anywhere except on a f*@king shortboard

Postby thedeadly » Mon Aug 08, 2005 4:54 pm

it also means that the wave fairies obviously live in or a short taxi ride from newquay :D
User avatar
thedeadly
 
Posts: 10765
Joined: Thu Jan 01, 1970 12:00 am
Location: ! No Signal !

Postby blind » Mon Aug 15, 2005 2:44 pm

thedeadly wrote:it also means that the wave fairies obviously live in or a short taxi ride from newquay :D
come on Deadly we all know you dont get fairys in Cornwall , you get Piskies , :wink: :wink:
User avatar
blind
 
Posts: 1298
Joined: Thu Jan 01, 1970 12:00 am
Location: Plymouth

Postby A77 » Tue Nov 22, 2005 4:04 pm

in response to the original query; both Ben and EJay are right; in deep water conditions the velocity of a monochromatic wave is equal to (g x T)/(2 x Pi), (where T is the wave period) taking g as 9.81 and Pi as 3.141 you get the simple approximation (as stated by Ben) as V = 1.5 x T.

In continuation; wave length (L) of a deep water wave (given V = L/T) is then equal to (g x T^2)/(2 x Pi), therefore, as Ben stated corrrectly: in deep water=> long waves travel faster than short waves AND longer length waves have longer periods. There is no confusion going on- just length and period are inextricably linked through the noted ratio. similar to time and space......

for those that are interested- the wave velocity changes it dependence in shallower water. in water depths < L/4 the velocity is given as the square root of (g x D), (where D is the water depth)- giving the approximation that the velocity is roughly three times the square root of the water depth. so it no longer depends on the wave period. at water depths between L/4 and L/2 the wave velocity is influenced by both period and water depth. given that a 9s wave has a wavelength of ~120m and most the north sea is shallower than 60m- as it also is for most the continental shelf off the SW coastline, most wave velocities we are likely to encounter surfing around the UK will be more depth dependent rather than period dependent. sadly.
A77
 
Posts: 18
Joined: Tue Nov 22, 2005 3:28 pm

Postby KFPC » Sat Jul 08, 2006 10:14 pm

The last time I checked the literature there were no handy (quick and dirty) formulas which would provide an estimate of the flow of water up the face of a breaking wave. (If anybody has one please pass it on.)

I understand what has been written so far (regarding a shoaling wave), and agree that its not unreasonable to extend the treatment a little further (to the point where the wave is about to break), however I've never been satisfied with the numbers I've gotten from doing this.

An estimate of the flow of water up the face (of a breaking wave) would be very nice to have, for if you have an estimate of the flow you could estimate the forces available. Here of course, the flow is a vector quantity, and my approach has been to resolve it two directions; one, in the direction of propagation, the other in the vertical direction. The shoaling velocity (described in prior posts) offers a reasonable way to estimate of the propagation component of the flow, its the vertical component that I'd love to be able to estimate.

Kevin
KFPC
 
Posts: 24
Joined: Thu Jul 06, 2006 9:29 am


Return to Surf Science

Who is online

Users browsing this forum: No registered users and 1 guest